Fat cells store excess energy and signal these levels to the brain. In a new study this week in Nature Medicine, Georgios Paschos PhD, a research associate in the lab of Garret FitzGerald, MD, FRS director of the Institute for Translational Medicine and Therapeutics, Perelman School of Medicine, University of Pennsylvania, shows that deletion of the clock gene Arntl, also known as Bmal1, in fat cells, causes mice to become obese, with a shift in the timing of when this nocturnal species normally eats. These findings shed light on the complex causes of obesity in humans.